Odds change for a couple of reasons, but, once again you need to understand how sportsbooks make money. Essentially a sportsbook looks to have an equal amount of risk bet on all outcomes of the wager. That is the ideal result for them. To ensure this, they adjust the odds to make a bet more attractive to the side with less bets.

Let’s take our example of coin flipping again, to keep things simple. Let’s also assume we have 5 bettors all looking to bet $100.

Heads Odds | Tails Odds | Heads Wins | Tails Wins | |

Bet 1 | -110 | -$91.92 | $100.00 |

If we look at “Bet 1” (below), the first bettor risks $100 on Heads for a chance to win $91.92. From the sportsbook’s point of view they are at risk of losing $91.92 on a Heads win, or retaining $100 on a tails win. Not ideal for a sportsbook since they look expect to make money on every event.

Heads Odds | Tails Odds | Heads Wins | Tails Wins | |

Bet 1 | -110 | -$91.92 | $100.00 | |

Bet 2 | -110 | $9.08 | $9.08 |

When Bet 2 is placed on Tails, the sportsbook gets an ideal outcome: no matter who wins, the book pays out $91.92 using the losers money, and retains $9.08 itself.

Heads Odds | Tails Odds | Heads Wins | Tails Wins | |

Bet 1 | -110 | -$91.92 | $100.00 | |

Bet 2 | -110 | $9.08 | $9.08 | |

Bet 3 | -110 | $108.08 | -$83.84 |

When Bet 3 is placed the sportsbook will likely not make an odds change to the -110, and they will still be available. By choosing Tails again, the risk to the sportsbook is again in the red. If heads wins, the book will have only one bettor to pay, but 2 losers and therefore would win $108.08. if Tails wins they are at risk $83.84. Definitely not what the sportsbook wants, but you’ll notice that the the overall risk is going down versus after Bet 1.

Heads Odds | Tails Odds | Heads Wins | Tails Wins | |

Bet 1 | -110 | -$91.92 | $100.00 | |

Bet 2 | -110 | $9.08 | $9.08 | |

Bet 3 | -110 | $108.08 | -$83.84 | |

Bet 4 | -110 | $208.08 | -$175.76 |

Bet 4 is where the sportsbook would get concerned. A third Tails bettor means they could potentially lose $175.76. Remember that the sportsbook isn’t gambling. They are expected to make money. That is why they would look to make an odds change to try to even out their risk.

Heads Odds | Tails Odds | Heads Wins | Tails Wins | |

Bet 1 | -110 | -$91.92 | $100.00 | |

Bet 2 | -110 | $9.08 | $9.08 | |

Bet 3 | -110 | $108.08 | -$83.84 | |

Bet 4 | -110 | $208.08 | -$175.76 | |

Bet 5 | +180 | $28.08 | -$75.76 |

To do this the sportsbook releases a line that is favorable to Heads, in this case, around +180. That means that a $100 bet on heads pays $180. On coin flipping, take those odds all day! For the book, that means that if Heads wins, they still have $28.08 profit and if Tails wins, the risk, though still negative is a lower -$75.76.

You can see that as this trend continues, eventually the sportsbook takes no risk and takes profit regardless of the result.

Something to keep in mind about this example is that it is unusual to see lines move with only a small number of bettors or dollars, but you can see that after thousands of people bet on a single event eventually profit is guaranteed and predictable.